Maybe rework subcommands for silo- and fleet-scoped timeseries

[david-crespo]

See fc95ef8

I noticed this while bumping the CLI in the v12 docs PR: the timeseries command hierarchy is a little confusing. It's all under experimental (which makes it less bad and we can probably punt for a while) but we're reproducing the system/developer distinction inside of experimental.

I think it's tolerable, but it's worth mulling over. Presumably these commands will eventually stop being experimental. In that case we have oxide timeseries (silo-scoped) and oxide system timeseries (fleet-scoped). That matches the API routes, but it's not really how we handle fleet-scoped stuff in the CLI. For example, silo stuff is all admin-oriented but it lives under oxide silo.

I'm thinking oxide timeseries is the top-level thing, and then we distinguish system vs. not inside there? It's still a little dicey, e.g., is it oxide timeseries system query vs. oxide timeseries query?